Variable Interest Entities (Summary Of Nonconsolidated VIEs Assets And Liabilities) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Variable Interest Entity [Line Items]
Insurance Loss Recoverable	$ 3,316		$ 3,046
Collateralized Debt Obligations [Member]
Variable Interest Entity [Line Items]
VIE Assets	17,552		26,507
Maximum Exposure to Loss	11,053		15,466
Investments			42	[1]
Premiums receivable	67	[2]	67	[2]
Insurance Loss Recoverable	3	[3]
Unearned premium revenue	59	[4]	58	[4]
Loss and Loss Adjustment Expense Reserves	78	[5]	3	[5]
Derivative liabilities	85	[6]	113	[6]
Residential Mortgage-Backed Agency [Member]
Variable Interest Entity [Line Items]
VIE Assets	36,464		47,669
Maximum Exposure to Loss	13,578		16,379
Investments	9	[1]	25	[1]
Premiums receivable	75	[2]	87	[2]
Insurance Loss Recoverable	2,967	[3]	2,773	[3]
Unearned premium revenue	73	[4]	86	[4]
Loss and Loss Adjustment Expense Reserves	485	[5]	428	[5]
Derivative liabilities	4	[6]	5	[6]
Commercial Mortgage-Backed [Member]
Variable Interest Entity [Line Items]
VIE Assets	4,751		5,001
Maximum Exposure to Loss	2,456		2,644
Premiums receivable	2	[2]	2	[2]
Unearned premium revenue	2	[4]	2	[4]
Consumer Asset-Backed [Member]
Variable Interest Entity [Line Items]
VIE Assets	6,372		8,015
Maximum Exposure to Loss	3,390		4,563
Investments	11	[1]	16	[1]
Premiums receivable	20	[2]	26	[2]
Unearned premium revenue	20	[4]	25	[4]
Loss and Loss Adjustment Expense Reserves	16	[5]	23	[5]
Corporate Asset-Backed [Member]
Variable Interest Entity [Line Items]
VIE Assets	22,006		29,855
Maximum Exposure to Loss	11,261		15,577
Investments			241	[1]
Premiums receivable	144	[2]	192	[2]
Insurance Loss Recoverable	21	[3]	22	[3]
Unearned premium revenue	159	[4]	205	[4]
Derivative liabilities	1	[6]	1	[6]
Total Global Structured Finance [Member]
Variable Interest Entity [Line Items]
VIE Assets	87,145		117,047
Maximum Exposure to Loss	41,738		54,629
Investments	20	[1]	324	[1]
Premiums receivable	308	[2]	374	[2]
Insurance Loss Recoverable	2,991	[3]	2,795	[3]
Unearned premium revenue	313	[4]	376	[4]
Loss and Loss Adjustment Expense Reserves	579	[5]	454	[5]
Derivative liabilities	90	[6]	119	[6]
Global Public Finance [Member]
Variable Interest Entity [Line Items]
VIE Assets	46,795		42,106
Maximum Exposure to Loss	21,578		21,774
Premiums receivable	216	[2]	215	[2]
Unearned premium revenue	268	[4]	270	[4]
Total Insurance [Member]
Variable Interest Entity [Line Items]
VIE Assets	133,940		159,153
Maximum Exposure to Loss	63,316		76,403
Investments	20	[1]	324	[1]
Premiums receivable	524	[2]	589	[2]
Insurance Loss Recoverable	2,991	[3]	2,795	[3]
Unearned premium revenue	581	[4]	646	[4]
Loss and Loss Adjustment Expense Reserves	579	[5]	454	[5]
Derivative liabilities	$ 90	[6]	$ 119	[6]

[1] Reported within "Investments" on MBIA's consolidated balance sheets.

[2] Reported within "Premiums receivable" on MBIA's consolidated balance sheets.

[3] Reported within "Insurance loss recoverable" on MBIA's consolidated balance sheets.

[4] Reported within "Unearned premium revenue" on MBIA's consolidated balance sheets.

[5] Reported within "Loss and loss adjustment expense reserves" on MBIA's consolidated balance sheets.

[6] Reported within "Derivative liabilities" on MBIA's consolidated balance sheets.